1. Field of the Invention
The present invention is directed to a method for recovering argon from an argon-oxygen-decarburization process waste gas. More particularly, the present invention is directed to a method for recovering argon from a feed mixture comprising argon, carbon monoxide, carbon dioxide, nitrogen, and optionally hydrogen and oxygen.
2. Description of the Prior Art
The commercial preparation of argon by separation of air is well known in the art. Air separation also produces equivalent quantities of nitrogen and oxygen. Because the demand for argon has been growing at a more rapid rate than the demand for nitrogen and oxygen, there is a need for alternative sources for producing argon. One such alternative source is the argon-oxygen-decarburization process waste gas from a steel making plant.
Steel manufacturing processes employ mixtures of oxygen and argon to act as an inert shield (inerting agent) during casting and to decrease the carbon monoxide partial pressure in the argon-oxygen-decarburization (AOD) process. Because argon is expensive, is used in large amounts during steel making, and is unchanged in the AOD process, recovering the argon-rich AOD process waste gas for use in this or other processes is desirable. Conventional methods for recovering argon from AOD process waste gas have generally focused on cryogenic processes which consist of pretreating the feed mixture to remove dust and other impurities, removing carbon dioxide and water from the mixture, and cryogenically distilling the gas to separate hydrogen, nitrogen, and carbon monoxide as distillate products and argon as the bottoms product in the cryogenic column.
A dedicated air separation plant, which has an argon purification unit, is generally employed to provide oxygen and argon to the steel mill for the AOD process and nitrogen as an inerting agent. An air separation plant typically consists of a double column (upper and lower) for separating air into oxygen and nitrogen products. Air is introduced into the bottom of the lower column which operates at a pressure of approximately 90 psia. The gaseous nitrogen product is typically withdrawn as a distillate product from the upper column which operates at low pressure, for example, at about 18 psia. A pure liquid nitrogen stream distillate product from the lower column is subcooled by expansion and used to provide reflux for the upper column. The oxygen product is withdrawn as a bottoms product, from the bottom of the upper column. An oxygen-rich stream is withdrawn as a bottoms product, from the bottom of the lower column, and admitted as feed gas to the upper column for further processing. An argon-rich stream typically containing about 10% to 12% argon admixed with a small amount of nitrogen and a significant amount of oxygen is withdrawn from the upper column at a location intermediate between the feed inlet to the column and the oxygen product outlet. This argon-rich stream is processed in a crude argon column which removes most of the oxygen to provide a 98% crude argon product. The oxygen-rich product from the crude argon column is recycled to the upper column. The 98% crude argon product can be further purified by removing oxygen by catalytic oxidation after addition of hydrogen, drying to remove the water formed in the catalytic oxidation unit, and purifying to remove the residual nitrogen in a cryogenic separation column.
Argon recovery methods are disclosed in more detail in "Cryogenic Processes and Equipment 1982", AlChE Symposium Series, No. 224, vol. 79, p. 12, Helmut Springmann, "Methods for Argon Recovery to Meet Increased Demand on the Argon Market", which disclosure is incorporated herein by reference.
U.S. Pat. Nos. 4,687,498, 4,750,925, and 4,752,311, issued to MacLean et al., disclose the recovery of argon from ammonia plant purge gases which contain hydrogen, nitrogen, methane, and argon. The methods comprise removing methane and nitrogen from the purge gas mixture by pressure swing adsorption, then removing hydrogen, and finally cryogenically distilling the mixture to remove remaining amounts of nitrogen and hydrogen thereby preparing a pure argon product.
In a pressure swing adsorption system (PSA), a gaseous mixture is passed at an elevated pressure through a bed of an adsorbent material which selectively adsorbs one or more of the components of the gaseous mixture. Product gas, enriched in the unadsorbed gaseous component(s), is then withdrawn from the bed.
The term "gaseous mixture", as used herein, refers to a gaseous mixture, such as air, primarily comprised of two or more components having different molecular size. The term "enriched gas" refers to a gas comprised of the component(s) of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through the adsorbent bed. The enriched gas generally must meet a predetermined purity level, for example, from about 90% to about 99%, in the unadsorbed component(s). The term "lean gas" refers to a gas exiting from the adsorption bed that fails to meet the predetermined purity level set for the enriched gas. When the strongly adsorbed component is the desired product, a cocurrent depressurization step and a cocurrent purge step of the strongly adsorbed component are added to the process.
The term "adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end of a single bed system is the inlet end of the single bed while the inlet end of the two bed system (arranged in series) is the inlet end of the first bed in the system. The outlet end of a single bed system is the outlet end of the single bed and the outlet end of the two bed system (arranged in series) is the outlet end of the second bed in the system. By using two adsorption beds in parallel in a system and by cycling (alternating) between the adsorption beds, product gas can be obtained continuously.
As a gaseous mixture travels through a bed of adsorbent, the adsorbable gaseous components of the mixture enter and fill the pores of the adsorbent. After a period of time, the composition of the gas exiting the bed of adsorbent is essentially the same as the composition entering the bed. This period of time is known as the breakthrough point. At some time prior to this breakthrough point, the adsorbent bed must be regenerated. Regeneration involves stopping the flow of gaseous mixture through the bed and purging the bed of the adsorbed components generally by venting the bed to atmospheric or subatmospheric pressure.
A pressure swing adsorption system generally employs two adsorbent beds operated on cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is in the adsorption or production step, the other bed is in the regeneration step. The two adsorption beds may be connected in series or in parallel. In a serial arrangement, the gas exiting the outlet end of the first bed enters the inlet end of the second bed. In a parallel arrangement, the gaseous mixture enters the inlet end of all beds comprising the system. Generally, a serial arrangement of beds is preferred for obtaining a high purity gas product and a parallel arrangement of beds is preferred for purifying a large quantity of a gaseous mixture in a short time cycle.
Between the adsorption step and the regeneration step, the pressure in the two adsorption beds is generally equalized by connecting the inlet ends of the two beds together and the outlet ends of the two beds together. During the pressure equalization step, the gas within the pores of the adsorption bed which has just completed its adsorption step (under high pressure) flows into the adsorption bed which has just completed its regeneration step (under low pressure) because of the pressure differential which exists between the two beds. The adsorption bed which completed its adsorption step is depressurized and the adsorption bed which completed its regeneration step is repressurized. This pressure equalization step improves the yield of the product gas because the gas within the pores of the bed which has just completed its adsorption step has already been enriched. When more than two beds are employed in the adsorption system, it is common to have a number of pressure equalizations steps.
Gas separation by the pressure swing adsorption method is more fully described in, for example, "Gas Separation by Adsorption Processes", Ralph T. Yang, Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes" Butterworth 1987, and in U.S. Pat. Nos. 2,944,627, 3,801,513, and 3,960,522, which disclosures are incorporated by reference herein. Modifications and improvements in the pressure swing adsorption process and apparatus are described in detail in, for example, U.S. Pat. Nos. 4,415,340 and 4,340,398, which disclosures are incorporated by reference herein.
While the above methods provide processes for recovering argon from waste gases, none of the methods disclose a satisfactory process for recovering argon from an argon-oxygen-decarburization process waste gas. Cryogenic separation processes tend to have a high capital cost especially when more than one pure product is required. Argon sources which contain a high concentration of argon (higher than 6%) are particularly attractive sources because of the high value of argon and because such sources provide an opportunity for cost-effective argon recovery. The present invention provides an improved method for recovering argon from argon-rich process waste gases such as AOD process waste gases which employs a combination of non-cryogenic and cryogenic separating steps. The present invention also provides a novel pressure swing adsorption method to remove carbon monoxide, carbon dioxide, and all or most of the nitrogen from argon in the process waste gas exiting a steel manufacturing plant.